Currently, global demands for high quality gasoline are increasingly stepped up. Technologies of producing more high quality gasoline are rapidly developed, but the development of technologies of producing high cetane number diesel is relatively slow. On the whole, the growth speed of the global demands for diesel will gradually go beyond that on gasoline, although the area demands vary. Diesel produced by the conventional catalytic cracking technology has a relatively lower cetane number, so that it is usually used as the harmonic component of diesel. In order to satisfy the demands for high quality diesel, it is necessary to modify the catalytic light diesel.
In the prior art, the processes for modifying the catalytic light diesel primarily include hydrogenation and alkylation. CN1289832A also discloses a process for modifying the catalytically cracked diesel, comprising, under the hydrogenation conditions, passing the feedstocks in turn through the hydrorefining catalyst and the hydrocracking catalyst in single-stage series without any intermediate separation. Due to said process, the cetane number of the diesel fraction in the product is increased by 10 units as compared with the feedstocks, and the sulfur content and nitrogen content are notably decreased.
CN1900226A discloses a catalytic cracking promoter for producing more diesel and a process for preparing the same. By the addition of a certain amount of such promoter, the diesel yield of FCC catalytic unit will be increased, and the product distribution will be improved without any change of the catalyst initially used in the refining unit. However, such process does not mention any improvement in the properties of diesel.
Low-carbon olefins, such as propylene and the like, are the important organic chemical materials, and propylene is the synthetic monomer for the products such as polypropylene, acrylonitrile and the like. With the rapid increase of the demands for the derivatives such as polypropylene, the demand for propylene is also increased annually. The global propylene demand has been increased from 15,200,000 tons twenty years ago to 51,200,000 tons in 2000, and the annual rate of increase is 6.3%. It is predicted that the propylene demand in 2010 will reach to 86,000,000 tons, and the annual rate of increase is about 5.6%.
The methods for producing propylene are primarily steam cracking and fluid catalytic cracking (FCC). Steam cracking involves thermally cracking light oils as the feedstocks, such as naphtha and the like, to produce ethylene and propylene, but the propylene yield is only about 15 wt %. FCC involves using as the feedstocks heavy oils vacuum gas oil (VGO). At present, 61% of propylene in the world is derived from the by-products obtained by producing ethylene via steam cracking; 34% is derived from the by-products obtained by producing gasoline and diesel via FCC in the oil refinery; and a small amount (about 5%) of propylene are obtained by propane dehydrogenation and ethylene-butylene metathesis reaction.
If the conventional routes for producing ethylene and propylene by steam cracking are still utilized, the petrochemical industry will be limited by several main restricting factors such as lack of light feedstock oils, undercapacity and unduly high cost.
Due to the advantages such as high adaptability and flexible operation of the feedstocks, more attentions are focused on FCC. In U.S.A, almost one half of the demand in the propylene market is derived from FCC unit. Technologies for improving the catalytic cracking for increasing the production of propylene are rapidly developing.
U.S. Pat. No. 4,422,925 discloses a process for contacting many hydrocarbons having different cracking properties with a hot regenerated catalyst and converting, wherein the hydrocarbons at least comprise a gas alkane feedstock and a liquid hydrocarbon feedstock. On the basis that different hydrocarbon molecules have different cracking properties, the reaction zone in the process is divided into many reaction zones for cracking, so as to produce more low molecule olefins.
CN1279270A discloses a catalytic conversion process for producing more diesel and liquefied gas, wherein the process is conducted in a four-stage riser or a fluidized bed reactor; the gasoline feedstocks, conventional cracking feedstocks and reaction terminating agent are fed into different positions. Said process can simultaneously increase the yields of liquefied gas and diesel, but it has relatively higher dry gas and coke yields.
For a long time, those ordinarily skilled in the art are of the opinion that, the higher the conversion ratio of the heavy oil catalytic cracking is, the better. Upon creative thinking and repeated experiments, the applicant finds that, it is not “the higher, the better” in the conversion ratio of the heavy oil catalytic cracking. When the conversion ratio reaches to a certain extent, the target product increases a little, while the dry gas and coke yields greatly increase. In the conventional catalytic cracking catalyst, the particles having a particle size of less than 40 μm are in an amount of about 20% by volume. Upon studies, it can be found that, although the small catalyst particles have higher cracking capability, the selectivity thereof to dry gas and coke is relatively worse. The optimization of the size distribution of the catalyst may improve the selectivity of dry gas and coke.
In order to efficiently utilize inferior heavy oil resources and satisfy the increasing demands for light fuel oils, it is necessary to develop a catalytic conversion process for converting heavy feedstocks into clean diesel and propylene in a great amount.